The development of electronic telephone station sets has heralded the introduction of many telephony features heretofore not readily available to the general public. Some examples of these features are repetory dialing, last number redial and saved number redial. Each of these features requires that dial tone be present on the telephone line before the feature is invoked. Otherwise a premature transmission of dialing digits to an associated telephone central office exchange will result in an incompletely dialed number and failure to achieve connection to a desired party. Many such features include a time delay before dialing such that in typical operation adequate time is provided between the OFF HOOK condition and the commencement of dialing. However, as is observed by Henry P. Sousa in U.S. Pat. No. 3,959,600 issued with the title "Remotely Programmable Call Diverter" on May 25, 1976, "Prior art devices have also failed to function well in a variety of telephone environments because of simple timing problems. As an example, the time it takes a telephone central exchange to furnish a dial tone to a subscriber varies widely from exchange to exchange, partially as a function of the equipment in the exchange and partially as a function of the volume of calls being handled at the time. Some prior art devices have initiated dialing solely as a function of time, and lost calls because of the failure to wait for a dial tone."
Of course the obvious solution is to provide the featured telephone station set with a dial tone detector such that receipt of dial tone is assured before dialing is commenced. The requirements of such a dial tone detector are that of detecting dial tone of a minimal continuous duration of about 1/4 second or more, and the detected dial tone must be distinguished from other intermittent tones within a dial tone passband. The dial tone passband must be broad enough to include at least the following types of dial tone if the feature telephone set is to be useable without restriction by various operating telephone companies. For example, in North America, at least three dial tone signal standards are in use. These are:
(1) Precise dial tone used on more recent switches and private branch exchanges. This is 350 hertz and 440 hertz combined;
(2) 600 hertz modulated by 120 hertz or 133 hertz. This is used on step-by-step switches, and so is an application for pulse dialing; and
(3) 400 hertz single frequency, which is used by some common carriers.
In a dial tone detector using a typical analog filter, a passband covering a range of at least 350 hertz to 733 hertz is required to detect and respond to any and all of these dial tone signals. Such an analog filter has disadvantages in that it can pass energy generated by DTMF signalling and requires bulky reactive components for its construction.
It is an object of the invention to provide dial tone detection with very sharply defined passband limits such that any of these dial tones can be detected without erroneously detecting DTMF signalling.
It is also an object of the invention to detect any of these dial tone occurrences within a fraction of a second, such that no undue or irksome delay of function is attributable to the telephone station set.